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Datasheet

Hydrilla verticillata (hydrilla)

Summary

  • Last modified
  • 22 November 2017
  • Datasheet Type(s)
  • Invasive Species
  • Pest
  • Host Plant
  • Preferred Scientific Name
  • Hydrilla verticillata
  • Preferred Common Name
  • hydrilla
  • Taxonomic Tree
  • Domain: Eukaryota
  •   Kingdom: Plantae
  •     Phylum: Spermatophyta
  •       Subphylum: Angiospermae
  •         Class: Monocotyledonae
  • Summary of Invasiveness
  • H. verticillata is a submerged plant that has rapid growth and a highly effective survival strategy that makes it one of the most troublesome aquatic weeds of water bodies in the world. It forms dense masses, o...

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Pictures

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PictureTitleCaptionCopyright
a, Leaf; b, spathe filled with male flowers; c, empty spathe; d, free-floating male flower; e, female flower; f, fruit; g, seed.
TitleH. verticillata - line drawing
Captiona, Leaf; b, spathe filled with male flowers; c, empty spathe; d, free-floating male flower; e, female flower; f, fruit; g, seed.
CopyrightSEAMEO-BIOTROP
a, Leaf; b, spathe filled with male flowers; c, empty spathe; d, free-floating male flower; e, female flower; f, fruit; g, seed.
H. verticillata - line drawinga, Leaf; b, spathe filled with male flowers; c, empty spathe; d, free-floating male flower; e, female flower; f, fruit; g, seed.SEAMEO-BIOTROP

Identity

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Preferred Scientific Name

  • Hydrilla verticillata (L. f.) Royle

Preferred Common Name

  • hydrilla

Other Scientific Names

  • Elodea verticillata F.Muell. 1888
  • Hydrilla alternifolia
  • Hydrilla angustifolia Hassk. 1840
  • Hydrilla dregeana Presl 1844
  • Hydrilla japonica
  • Hydrilla lithuanica (Besser) Dandy 1952
  • Hydrilla muscoides Planch. 1849
  • Hydrilla najadifolia Zoller & Mortizi 1846
  • Hydrilla polysperma Blatt. 1931
  • Hydrilla subulata Royle
  • Hydrilla wightii Planch. 1849
  • Udora verticillata Spreng. 1824
  • Vallisneria verticillata Roxb. 1814

International Common Names

  • English: esthwaite waterweed; water thyme; waterthyme
  • Spanish: maleza acuática
  • French: Hydrilla de Lithuanie

Local Common Names

  • Cuba: hidrila
  • Germany: Guinduetzel; Quirllboettrige; Wasserquirl
  • Japan: kinomo

EPPO code

  • HYLLI (Hydrilla lithuanica)
  • HYLVE (Hydrilla verticillata)

Summary of Invasiveness

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H. verticillata is a submerged plant that has rapid growth and a highly effective survival strategy that makes it one of the most troublesome aquatic weeds of water bodies in the world. It forms dense masses, outcompeting native plants and interfering with many uses of waterways. It can be spread by water flow, waterfowl and recreational activities and is sold as an aquarium plant. In the USA it has been listed as Federal Noxious Weed since 1976; its import is prohibited in Western Australia and Tasmania, and it is on the EPPO alert list.

Taxonomic Tree

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  • Domain: Eukaryota
  •     Kingdom: Plantae
  •         Phylum: Spermatophyta
  •             Subphylum: Angiospermae
  •                 Class: Monocotyledonae
  •                     Order: Hydrocharitales
  •                         Family: Hydrocharitaceae
  •                             Genus: Hydrilla
  •                                 Species: Hydrilla verticillata

Notes on Taxonomy and Nomenclature

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Hydrilla is a monotypic genus; H. verticillata is both phenotypically and genetically variable, but Cook and Lüönd (1982) do not recognize any intraspecific taxa (Preston and Croft, 1997). There are marked differences in isoenzyme patterns among strains from different regions (Verkleij and Pieterse, 1991); dioecious plants showed variation in phenoptyes of diploid (2n=16) and triploid (2n=24) accessions, however monoecious plants showed no such variation and are assumed to be ramets of the same clone (Nakamura et al., 1998). In certain cases some specific isoenzyme phenotypes can be designated as diagnostic for strains from a particular region.  Material from Connecticut in the north-eastern USA, is triploid and dioecious (Les et al., 1997) suggesting that it originated from a single introduction, however both diploid and triploid; and monoecious and dioecious strains occur in the USA, suggesting repeated introductions. Most introductions are likely to derive from the aquarium trade (Madeira et al., 2000).

Description

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H. verticillata is a submerged, monoecious or dioecious perennial. Its stems are branched, about 1 mm thick and up to 3 m long; the internodes are 3 to 50 mm long. The sessile leaves are formed in whorls at the nodes; there are 3-8, sometimes up to 12 leaves in a whorl. The leaves are 7-40 mm long, linear to lanceolate, with a conspicuous midrib. They have sharply toothed margins and spines on the vein on the lower side of the leaves; a few teeth may also be formed on this vein. These leaf characteristics are commonly used to distinguish H. verticillata from similar submerged plants in the Hydrocharitaceae, like Egeria and Elodea spp.

The inflorescences are unisexual, arising from spathes situated in the leaf axils, each flower has three sepals and three petals. All six perianth parts are clear or translucent green (the sepals usually slightly reddish).The male spathe is about 1.5 mm long, solitary in the leaf axils, somewhat spiny. The female spathe is about 5 mm long, solitary in the leaf axils. There are three petals, three stamens and three styles. The ovary is cylindrical to narrowly conical and is enclosed in the base of a hypanthium; the style is as long as the hypanthium and there are three stigmas. For further information, see Cook et al. (1974) and Aston (1977).

The fruit is cylindrical, about 7 mm long and 1.5 mm wide. It contains 2-7 oblong-elliptic seeds. For further information, see Cook and Lüönd (1982); Swarbrick et al. (1981); and Yeo et al. (1984).

In order to survive conditions which are unfavourable for growth, the plant produces two types of special hibernating organs. These structures are respectively formed in the axil of a leaf (generally described as axillary turions, turions or green turions) and at the tip of branches which grow into the hydrosoil (generally described as subterranean turions, brown turions or tubers). In 1983, it was proposed that henceforth these structures should be called axillary turion and subterranean turion (Pieterse, 1983). Both structures, which are anatomically and morphologically similar, can be considered as dormant apices or turions, i.e. short, specialized shoots of aquatic plants in which food material is stored and which eventually become detached from the mother plant. The axillary turions are stalked, cylindrical or slightly conical in shape, 3 to 12 mm long and 2 to 3.5 mm wide. Axillary turions are frequently formed on free-floating fragments. The subterranean turions are boat-shaped, 4 to 15 mm long and 2.5 to 6 mm wide, and covered by 16 to 17 whorls of tough and fleshy scale leaves.

Plant Type

Top of page Annual
Aquatic
Herbaceous
Perennial
Vegetatively propagated

Distribution

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H. verticillata has a wide and rather disjointed geographical range (see Pieterse, 1981; Cook and Lüönd, 1982, Preston and Croft, 1997). This range includes South-East Asia, Australia, Central Africa, a few sites in Europe and, since at least the early 1960s, the USA and the Panama Canal area. In Asia, it is found from Iran and Afghanistan through Pakistan and India to South-East Asia, reaching northwards to Japan, Korea and Manchuria, China. The common dioecious type of H. verticillata is thought to originate from the Indian subcontinent, whereas the origin of the monoecious type is likely to be Korea (Madeira et al., 1997). It also occurs in the Moluccas, Indonesia and Papua New Guinea. In the Indian Ocean it occurs on Mauritius, Réunion and Madagascar. In the Pacific Ocean it has been found on Fiji and Guam. It is a common plant in the northern and western parts of Australasia and it also occurs in the North Island of New Zealand.

On the African continent it occurs around Lake Victoria and Lake Tanganyika in the Rift Valley of East Africa, while it has also been reported from Mozambique and a few isolated places in West Africa and, in 2006, from South Africa. It is thought to be native but is relatively rare in Europe (Preston and Croft, 1997), sufficiently so that it is protected in Lithuania (Balevicius, 1998). It occurs in certain areas in Poland and Belarus, and has been found in solitary lakes in Ireland (Preston and Croft, 1997).

At least three different strains of H. verticillata have spread to the USA and the Panama Canal area. The first record is of the dioecious strain in the early 1950s which was imported for use in aquariums; other strains were separate introductions (Jacono, 2011). At present it occurs throughout the Gulf States, in southern California, and in certain localities in the eastern  and central parts of the country. For a detailed description of the distribution in the USA, see Jacono et al. (2011). There are recent reports from South America (Brazil) and Caribbean islands.

Distribution Table

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The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Continent/Country/RegionDistributionLast ReportedOriginFirst ReportedInvasiveReferenceNotes

Asia

AfghanistanWidespreadCook and Lüönd, 1982
BangladeshRestricted distributionPieterse, 1981; Cook, 1996; EPPO, 2014
Brunei DarussalamPresentWaterhouse, 1993
CambodiaRestricted distributionPieterse, 1981; EPPO, 2014
ChinaRestricted distributionPieterse, 1981; EPPO, 2014
-HubeiPresentZhou et al., 1996
-JiangsuPresentNi, 1997; Zhu et al., 1993; Zhang et al., 1999
-YunnanWidespreadCook and Lüönd, 1982
IndiaRestricted distributionNativeBuckingham & Bennet, 1998; Pieterse, 1981; Cook and Lüönd, 1982; EPPO, 2014
-Andhra PradeshPresentCook, 1996
-Arunachal PradeshPresentCook, 1996
-AssamPresentCook, 1996
-BiharWidespreadBuckingham & Bennet, 1998; Cook and Lüönd, 1982
-DelhiPresentCook, 1996
-GoaPresentCook, 1996
-GujaratPresentBuckingham & Bennet, 1998; Haider et al., 1995
-HaryanaPresentCook, 1996
-Himachal PradeshPresentCook, 1996
-Indian PunjabPresentCook, 1996
-Jammu and KashmirPresentCook, 1996
-KarnatakaPresentCook, 1996
-KeralaPresentCook, 1996
-Madhya PradeshPresentCook, 1996
-MaharashtraWidespreadBuckingham & Bennet, 1998; Cook and Lüönd, 1982
-ManipurPresentCook, 1996
-MeghalayaPresentCook, 1996
-OdishaPresentCook, 1996
-RajasthanPresentBuckingham & Bennet, 1998; Usha-Pandey et al., 2002
-Tamil NaduPresentCook, 1996
-Uttar PradeshWidespreadBuckingham & Bennet, 1998; Cook and Lüönd, 1982
-West BengalWidespreadBuckingham & Bennet, 1998; Cook and Lüönd, 1982
IndonesiaRestricted distributionPieterse, 1981; Cook and Lüönd, 1982; EPPO, 2014
-MoluccasWidespreadCook and Lüönd, 1982
IranRestricted distributionCook and Lüönd, 1982; EPPO, 2014
IraqPresentIntroducedAl-Mandeel, 2013
JapanRestricted distributionEPPO, 2014
-HonshuWidespreadCook and Lüönd, 1982
Korea, DPRRestricted distributionEPPO, 2014
Korea, Republic ofRestricted distributionPieterse, 1981; Lee et al., 2001; EPPO, 2014
LaosWidespreadPieterse, 1981
LebanonRestricted distributionHolm et al., 1979; EPPO, 2014
MalaysiaWidespreadPieterse, 1981; Verkleij et al., 1983
MyanmarPresentWaterhouse, 1993
NepalRestricted distributionPieterse, 1981; Verkleij et al., 1983; Cook, 1996; Rai and Pradhan, 2000; EPPO, 2014
PakistanRestricted distributionBuckingham & Bennet, 1998; Pieterse, 1981; Cook, 1996; EPPO, 2014
PhilippinesRestricted distributionPieterse, 1981; EPPO, 2014
SingaporeWidespreadCook and Lüönd, 1982
Sri LankaRestricted distributionNativeSolangaarachchi & Perera, 2000; Pieterse, 1981; Cook, 1996; Wijeyaratne and Perera, 2000; EPPO, 2014
ThailandRestricted distributionPieterse, 1981; Siriworakul et al., 1997; EPPO, 2014
VietnamRestricted distributionPieterse, 1981; EPPO, 2014

Africa

BurundiRestricted distributionPieterse, 1981
Congo Democratic RepublicRestricted distributionPieterse, 1981
Côte d'IvoireRestricted distributionPieterse, 1981
KenyaRestricted distributionHolm et al., 1979; EPPO, 2014
MadagascarPresentIntroducedUSDA-ARS, 2011
MauritiusRestricted distributionCook and Lüönd, 1982; EPPO, 2014
MozambiqueRestricted distributionPieterse, 1981
RéunionRestricted distributionCook and Lüönd, 1982
South AfricaRestricted distributionIntroduced2006 Invasive Madeira et al., 2007
Spain
-Canary IslandsRestricted distribution Not invasive Cook and Lüönd, 1982; USDA-ARS, 2011
TanzaniaRestricted distributionHolm et al., 1979; EPPO, 2014
UgandaRestricted distributionPieterse, 1981; EPPO, 2014
ZambiaRestricted distributionPieterse, 1981

North America

MexicoRestricted distributionIntroducedCamarena & Aquilar, 1999; Rendon-Pimentil et al., 1996; Novelo and Martinez, 1989
USAWidespreadIntroduced Invasive Pieterse, 1981; Cook and Lüönd, 1982; EPPO, 2014
-AlabamaWidespreadIntroduced Invasive Pieterse, 1981; USDA-NRCS, 2011
-ArizonaPresentIntroduced Invasive USDA-NRCS, 2011
-ArkansasWidespreadIntroducedPieterse, 1981
-CaliforniaWidespreadIntroduced Invasive Pieterse, 1981; Spencer and Ksander, 1999; USDA-NRCS, 2011
-ConnecticutRestricted distributionIntroduced Invasive Les et al., 1997; USDA-NRCS, 2011
-DelawarePresentIntroducedUSDA-NRCS, 2011
-District of ColumbiaPresentIntroducedUSDA-NRCS, 2011
-FloridaWidespreadIntroduced Invasive Smither-Kopperl, 1999; Pieterse, 1981; Sutton and Portier, 1995; Fox et al., 1996; Sutton, 1996; Mataraza et al., 1999; Hanlon et al., 2000; Wheeler and Center, 2001; Cuda et al., 2002; USDA-NRCS, 2011
-GeorgiaWidespreadIntroducedPieterse, 1981; Brown and Maceina, 2002; USDA-NRCS, 2011
-IdahoRestricted distributionIntroduced2007 Invasive Jacono et al., 2011
-IndianaRestricted distributionIntroduced Invasive Alix et al., 2009; Jacono et al., 2011
-IowaPresentIntroducedUSDA-NRCS, 2011
-KansasRestricted distributionIntroduced2008Jacono et al., 2011
-KentuckyRestricted distributionIntroducedJacono et al., 2011
-LouisianaWidespreadIntroducedPieterse, 1981; Battle and Mihuc, 2000; USDA-NRCS, 2011
-MainePresentIntroduced Invasive USDA-NRCS, 2011
-MarylandPresentIntroducedUSDA-NRCS, 2011
-MassachusettsRestricted distributionIntroducedJacono et al., 2011
-MississippiWidespreadIntroduced Invasive Pieterse, 1981; USDA-NRCS, 2011
-New JerseyRestricted distributionIntroduced2003Jacono et al., 2011
-New YorkRestricted distributionIntroduced2008Jacono et al., 2011
-North CarolinaWidespreadIntroduced Invasive Harlan et al., 1985; Ryan et al., 1995; USDA-NRCS, 2011
-PennsylvaniaPresentIntroducedUSDA-NRCS, 2011
-South CarolinaWidespreadIntroduced Invasive Killgore et al., 1998; Kirk et al., 2000; Kirk et al., 2001; USDA-NRCS, 2011
-TennesseePresentIntroducedUSDA-NRCS, 2011
-TexasWidespreadIntroduced Invasive Pieterse, 1981; USDA-NRCS, 2011
-VirginiaWidespreadIntroducedSteward et al., 1984; Everitt et al., 1999; Doyle and Smart, 2001; Owens et al., 2001; USDA-NRCS, 2011
-WashingtonPresentIntroduced Invasive USDA-NRCS, 2011
-WisconsinPresent, few occurrencesIntroduced2007Jacono et al., 2011

Central America and Caribbean

BarbadosPresentIntroducedBroome et al., 2007
Costa RicaPresentIntroducedRojas and Agüero, 1996
CubaPresentIntroduced Invasive Oviedo Prieto et al., 2012
DominicaPresentIntroduced Invasive Jérémie, 1985
GrenadaPresent, few occurrencesIntroduced1991 Invasive Lemke and Roberts, 1993; Maddi, 2009
GuadeloupePresentIntroduced Invasive Maddi et al., 2008
GuatemalaRestricted distributionIntroduced Invasive Binimelis et al., 2007
MartiniquePresent, few occurrencesIntroduced2010 Invasive Maddi and Brizard, 2010
PanamaWidespreadIntroducedPieterse, 1981; Cook and Lüönd, 1982
Puerto RicoPresentIntroduced Invasive

South America

BrazilRestricted distributionIntroduced Invasive Sousa et al., 2009
VenezuelaPresentIntroducedSchotman, 1989

Europe

AustriaRestricted distributionIntroduced Not invasive Preston and Croft, 1997
BelarusRestricted distribution Not invasive Tutin et al., 1972; Pieterse, 1981
GermanyPresent Not invasive Tutin et al., 1972; Holm et al., 1979; Preston and Croft, 1997
IrelandRestricted distribution Not invasive Tutin et al., 1972; Holm et al., 1979; Cook and Lüönd, 1982; EPPO, 2014
LatviaRestricted distribution Not invasive Cook and Lüönd, 1982; EPPO, 2014
LithuaniaRestricted distribution Not invasive Balevicius, 1998; EPPO, 2014
PolandRestricted distribution Not invasive Cook and Lüönd, 1982; Klosowski and Tomaszewicz, 1997; Preston and Croft, 1997; EPPO, 2014
Russian FederationPresentEPPO, 2014
-Russian Far EastRestricted distribution Not invasive Probatova and Buch, 1981
SpainPresent Not invasive Holm et al., 1979
UKPresent, few occurrences Not invasive Holm et al., 1979; EPPO, 2014
-England and WalesPresent, few occurrencesEPPO, 2014
-ScotlandRestricted distribution Not invasive Lansdown & Darwell, 1999

Oceania

AustraliaRestricted distributionEPPO, 2014
-Australian Northern TerritoryWidespreadSainty and Jacobs, 1981
-New South WalesWidespreadSainty and Jacobs, 1981; Roberts et al., 2001
-QueenslandWidespreadBalcuinas & Burrows, 1996; Sainty and Jacobs, 1981; Hearnden and Kay, 1997
-South AustraliaWidespreadSainty and Jacobs, 1981
-VictoriaWidespreadSainty and Jacobs, 1981
-Western AustraliaWidespreadSainty and Jacobs, 1981
FijiRestricted distributionCook and Lüönd, 1982; EPPO, 2014
GuamWidespreadCook and Lüönd, 1982
New ZealandRestricted distributionHofstra et al., 2001; Cook and Lüönd, 1982; Winton et al., 1996; Hofstra et al., 1999; EPPO, 2014
Papua New GuineaWidespreadCook and Lüönd, 1982; Preston and Croft, 1997

Risk of Introduction

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H. verticillata poses a potential threat to areas outside its native habitats; this has been demonstrated in the USA and the Panama Canal area. As H. verticillata is introduced to the New World as an aquarium plant, legislative measures should be taken worldwide to restrain this trade.

Habitat

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In the tropics, H. verticillata is described as tolerant of a wide variety of water conditions, from acidic and oligotrophic to eutrophic or brackish; it thrives on many kinds of pollution and tolerates a great deal of disturbance (Cook and Lüönd, 1982), although increasing salinity appears to limit its dispersal (Rout et al., 1998; Mataraza et al., 1999; Rout and Shaw, 2001). Due to its tolerance of low light conditions (White et al., 1996), it is capable of growing in water up to 7 m deep (Yeo et al., 1984). In the tropics, it forms dense monospecific stands (Valley and Bremigan, 2002). In temperate regions, it grows in alkaline, moderately calcareous, mesotrophic or slightly eutrophic waters (Preston and Croft, 1997), richer in SO4, but generally poorer in Na, K and Cl than those of Elodea canadensis (Klosowski and Tomaszewicz, 1997). It also appears to occur more often as scattered stands within more diverse aquatic plant communities (Klosowski and Tomaszewicz, 1997; Balevicius, 1998).

Habitat List

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CategoryHabitatPresenceStatus
Brackish
Estuaries Secondary/tolerated habitat Harmful (pest or invasive)
Freshwater
Freshwater Present, no further details Harmful (pest or invasive)
Irrigation channels Principal habitat Harmful (pest or invasive)
Lakes Principal habitat Harmful (pest or invasive)
Ponds Present, no further details Harmful (pest or invasive)
Reservoirs Present, no further details Harmful (pest or invasive)
Rivers / streams Principal habitat Harmful (pest or invasive)
Terrestrial-natural/semi-natural
Wetlands Principal habitat Harmful (pest or invasive)

Hosts/Species Affected

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H. verticillata occurs in lowland irrigated and tidal ricefields in South-East Asia where it is most troublesome during the first half of the growth period of the crop.

Host Plants and Other Plants Affected

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Plant nameFamilyContext
Oryza sativa (rice)PoaceaeMain

Growth Stages

Top of page Seedling stage, Vegetative growing stage

Biology and Ecology

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H. verticillata is a submerged plant which is rooted by means of filiform, adventitious roots. The stems, which consist of distinct nodes and internodes, are branched and approach or touch the surface of the water. The internodes tend to elongate in flowing water. The flowers are unisexual, arising from spathes situated in the leaf axils, each flower has three sepals and three petals. All six perianth parts are clear or translucent green (the sepals usually slightly reddish). The ovary is enclosed in the base of a hypanthium, the style is as long as the hypanthium and there are three stigmas. Due to an elongation of the hypanthium, the female flower ascends to the surface of the water. The perianth segments remain closed over the stigmas during this movement and retain a bubble of air above them. The perianth segments open to form a wide funnel which floats with its rim just at the water surface, its walls holding back the water and preventing wetting of the stigmas. The male flower becomes detached from the plant and subsequently rises to the surface of the water where the perianth segments uncurl. The anthers dehisce explosively and spread pollen for some 20 cm around the open flower. Pollination occurs via the air.

H. verticillata spreads horizontally by means of branches which grow over the bottom of a waterbody. Vertical branches and roots are produced at nodes on these runners. Vegetative multiplication is also possible by means of fragmentation, i.e. pieces of branches which have become detached are able to form new, rooted plants, if they come into contact with a favourable substratum. In the USA, hydrilla grows optimally at 20-27°C.

It is capable of surviving conditions unfavourable for growth, by producing two types organ capable of remaining dormant for extended periods. These structures are respectively formed in the axil of a leaf (generally described as axillary turions, turions or green turions) and at the tip of branches which grow into the hydrosoil (generally described as subterranean turions, brown turions or tubers). (Turions can be defined as short, specialized shoots of aquatic plants in which food material is stored and which eventually become detached from the parent plant). The axillary turions are stalked and cylindrical or slightly conical in shape. The subterranean turions are boat-shaped and covered by whorls of tough and fleshy scale leaves. For further information on these turions, see the Description section. As many as 1000 (Pieterse, 1981) to 6000 (USDA, 2011) subterranean turions may be produced per square metre in one growing season and remain viable for over 4 years (USDA, 2011). In Florida, USA, the average number of subterranean turions varies from 36 to 207 per m² and the average number of axillary turions from 5 to 90 per m² (Sutton and Portier, 1985). In areas where H. verticillata dies during the winter, the formation of turions occurs mainly in the autumn. Axillary turions are frequently formed on free-floating fragments. The formation of subterranean turions is stimulated by short days (Steward and Van, 1987). 

There have been numerous studies into the biology of turion production; the most useful of these is a comprehensive review (Netherland, 1997). Additional studies have dealt with the effects of photperiod on turion development (Steward, 1997; Steward, 2000); factors affecting turion formation (Langeland et al., 1996); the size of turions (Spencer and Ksander, 1995); and the timing of plant development from turions (Spencer and Ksander, 1995; Spencer and Ksander, 1997).

H. verticillata may be either monoecious or dioecious. Its rapid vegetative growth and, as a consequence, the formation of large clones, questions whether strains which produce only male or female flowers are able to reproduce effectively by sexual means. In California and the Gulf States of the USA, and in Europe, there is no seed formation because only female flowers are produced.
 
For further information, see the Description section and Cook and Lüönd (1982), Swarbrick et al. (1981) and Yeo et al. (1984).

Natural enemies

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Natural enemyTypeLife stagesSpecificityReferencesBiological control inBiological control on
Bagous affinis Herbivore Florida; USA
Bagous hydrillae Herbivore
Bagous laevigatus Herbivore
Cricotopus tricinctus Herbivore not specific
Ctenopharyngodon idella Herbivore California; Florida; Indonesia; Texas
Cygnus atratus Herbivore Whole plant not specific
Fusarium crookwellense Pathogen
Fusarium culmorum Pathogen Whole plant not specific
Hirschmanniella caudacrena Parasite
Hydrellia balciunasi Herbivore Leaves/Stems
Hydrellia pakistanae Herbivore Leaves/Stems USA
Labeo rohita Herbivore Whole plant not specific
Macrophomina phaseolina Pathogen
Mycoleptodiscus terrestris Pathogen Mississippi
Oreochromis zillii Herbivore
Papulaspora aspera Pathogen
Parapoynx diminutalis Herbivore Leaves/Stems Thailand
Parapoynx seminealis Herbivore not specific
Plectosphaerella cucumerina Pathogen not specific
Pythium dissotocum Leaves not specific

Notes on Natural Enemies

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Natural enemies of H. verticillata include some insect species which have potential as biological control agents. A substantial list of insects which damage H. verticillata has been produced following extensive surveys in tropical Asia and Australia. Groups which show the greatest potential are: weevils, especially of the genus Bagous (O'Brien and Askevold, 1992; Balciunas, 1985; O'Brien, 1995), such as B. affinis (Godfrey and Anderson, 1994; Buckingham and Bennet, 1998) and B. hydrillae (Balciunas et al., 1996; Wheeler and Center, 1997; Van et al., 1998), which feed on the subterranean turions; leaf-mining and stem-boring ephydrid flies, primarily of the genus Hydrellia (O'Brien and Askevold, 1992; Balciunas, 1985), including H. pakistanae (Dray and Center, 1996; Wheeler and Center, 1996; Center et al., 1997; Van et al., 1998; Wheeler and Center, 2001; Doyle et al., 2002) and H. balcuinasi (Balciunas and Burrows, 1996; Grodowitz et al., 1997); and leaf-feeding aquatic pyralid moths, primarily of the genus Parapoynx (Balciunas, 1985; O'Brien and Askevold, 1992), such as P. seminealis (Buckingham and Bennet, 2001) and P. diminutalis (Buckingham and Bennet, 1996; Siriworakul et al., 1997).

The grass carp (Ctenopharyngodon idella), a phytophagous fish, feeds on many aquatic plants, in particular, submerged ones. It may be considered a natural enemy of H. verticillata, although invasive forms of the plant, which are adapted to a tropical climate, do not occur in its natural habitat in China and Siberia. At present, grass carp is the most promising biological control agent for H. verticillata (Kracko and Noble, 1993; Zhu et al., 1993; Rendón-Pimental et al., 1996; Rojas and Aguero, 1996; Sutton, 1996; Killgore et al., 1998; Camarena and Aguilar, 1999; Osborne and Riddle, 1999; Maceina et al., 1999; Hanlon et al., 2000; Kirk et al., 2000).

Other species which may be considered natural enemies of H. verticillata include: the midge Cricotopus lebetis [Cricotopus tricinctus] of unknown origin (Epler et al., 2000; Cuda et al., 2002); fungal pathogens such as Mycoleptodiscus terrestris (Shearer et al., 1996; Nelson et al., 1998; Shearer, 1998; Shearer and Nelson, 2002); Plectosporium tabacinum [Monographella cucumerina] (Smither-Kopperl et al., 1999b); and Fusarium culmorum (Smither-Kopperl et al., 1998; Smither-Kopperl et al., 1999a). In New Zealand, black swans (Cygnus atratus) have been recorded grazing on H. verticillata (Hofstra et al., 1999).

For further information on natural enemies of H. verticillata in Asia, see Gopal (1990).

Means of Movement and Dispersal

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In North America and elsewhere, H. verticillata has been introduced to rivers, ponds and canals as discarded fragements from aquariums or contaminants of aquatic garden plants. It is then mainly spread by recreational boats, on their motors and trailers etc. Stem pieces root in the substrate and develop into new colonies, commonly near boat ramps. Boat traffic shatters and spreads hydrilla throughout the water body (Jacono et al., 2011). 

Early reports of introductions in Europe are believed to be via the feathers or feet of waterfowl (EPPO, 2011). Tubers and turions can also survive ingestion and regurgitation by waterfowl (ISSG, 2011).

Pathway Causes

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CauseNotesLong DistanceLocalReferences
Digestion and excretion Yes
Pet trade Yes Yes

Pathway Vectors

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VectorNotesLong DistanceLocalReferences
Floating vegetation and debris Yes
Host and vector organisms Yes
Plants or parts of plants Yes Yes
Ship hull fouling Yes
Water Yes

Impact Summary

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CategoryImpact
Animal/plant collections None
Animal/plant products None
Biodiversity (generally) Negative
Crop production None
Environment (generally) None
Fisheries / aquaculture Negative
Forestry production None
Human health None
Livestock production None
Native fauna Negative
Native flora Negative
Rare/protected species Negative
Tourism Negative
Trade/international relations None
Transport/travel Negative

Economic Impact

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Due to its rapid growth and a highly effective survival strategy, H. verticillata is one of the most troublesome aquatic weeds in the world. It rapidly outcompetes other plant species and forms dense masses, which may completely fill the volume of waterbodies. Consequently, the often multifunctional use of canals, rivers and lakes becomes seriously hampered by infestations of the weed.

Harmful effects of H. verticillata include: impeding the movement of irrigation and drainage water; hindering navigation and recreational use of the water; physical interference with hydro-electric schemes and fisheries; competition with native plants; impacts on native fauna; reductions in size and weight of sport fish (Colle and Shireman, 1980 in Jacono et al., 2011); and the creation of favourable habitats for organisms which cause or transmit disease.

Although it is increasingly troublesome in its original habitat in South-East Asia and Australia, particularly in man-made lakes and irrigation canals, its impact is most significant where it is introduced. This applies, in particular, to the USA, where it was introduced in Florida in the early 1950s (Schardt, 1995). The costs of controlling H. verticillata in Florida were reported to be $200 per ha per year (Haller, 1995) when an area of more than 12,000 ha were heavily infested in the state. Useful summaries of economic and ecological costs due to H. verticillata are provided by the Northeast Aquatic Nuisance Species Panel  (for the USA) and by Hofstra and Champion (2006; for New Zealand). 

Environmental Impact

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Impact on Habitats

H. verticillata forms dense masses, which may completely fill the volume of waterbodies. Invasion often begins in deep dark waters where most plants cannot grow. Hydrilla grows aggressively and competitively, spreading through shallower areas and forming thick mats in surface waters that may become so deep that sunlight is blocked out (Jacono et al., 2011).

Infestation has been shown to alter the physical and chemical characteristics of lakes: affecting stratification of the water column (Schmitz et al., 1993; Rizzo et al., 1996), decreasing oxygen levels (Pesacreta, 1988; Miranda and Hodges, 2000), and impeding the movement of irrigation and drainage water (Jacono et al., 2011).

Impact on Biodiversity

H. verticillata outcompetes native aquatic plants. In southeast USA, it displaces native vegetation such as wild celery (Vallisneria americana) and coontail (Ceratophyllum demersum) (van Dijk, 1985; Rizzo et al., 1996 in Jacono et al., 2011).

It affects zooplankton and phytoplankton densities (Schmitz and Osbourne, 1984; Schmitz et al., 1993 in Jacono et al., 2011) and supports reduced invertebrate species diversity (Thorp et al., 1997). It has been implicated in reduced fish numbers, size and in fish kills (Rizzo et al., 1996 in Jacono et al., 2011). In a heavy infestation, predatory fish cannot hunt effectively; however ISSG (2011) notes a benefit to prey fish at <30% hydrilla cover.

Epiphytic cyanobacteria found on hydrilla are thought to be the agents producing a toxin that causes avian vacuolar myelinopathy (AVM) a disease that has killed at least 100 bald eagles (Haliaeetus leucocephalus) and thousands of American coots (Fulica americana) since 1994 in locations from Texas to North Carolina, USA (Wilde et al., 2005). The incidence of AVM is likely to increase as H. verticillata spreads.

Social Impact

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As H. verticillata rapidly forms such dense mats, it becomes impossible to use outboard motors and to pursue fishing, swimming and other recreational activities. It can also result in reduced water flow and stagnant pools which become habitats for mosquito larvae. A case study on the social impact of invasion of a lake in Guatemala by hydrilla has been produced by Binemelis et al. (2007).
 

Risk and Impact Factors

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Uses List

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Medicinal, pharmaceutical

  • Traditional/folklore

Diagnosis

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Although Hydrilla is generally regarded today as a monotypic genus, the species H. verticillata is very polymorphic, especially with regard to the size of the leaves and the thickness of the stems. It is, however, very difficult to distinguish the various strains as the morphology of these plants varies to a large extent with environmental factors. Chromosome numbers are not very diagnostic as in general only two cytodemes can be observed, 2n=16 and 2n=24. However, there are marked differences in isoenzyme patterns among strains from different regions (Verkleij and Pieterse, 1991). In certain cases some specific isoenzyme phenotypes can be designated as diagnostic for strains from a particular region.

Similarities to Other Species/Conditions

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Leaf characteristics are commonly used to distinguish H. verticillata from similar submerged plants in the Hydrocharitaceae, like Egeria and Elodea spp.

The sessile leaves of H. verticillata are formed in whorls at the nodes; there are 3-8, sometimes up to 12 leaves in a whorl. The leaves are 7-40 mm long, linear to lanceolate, with a conspicuous midrib. They have sharply toothed margins and spines on the vein on the lower side of the leaves; a few teeth may also be formed on this vein.

Prevention and Control

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Hydrilla can be controlled by physical, chemical and biological methods, or by a combination of these methods (integrated pest management).

Physical Control

The removal of plants either manually, using hand tools, or mechanically, using machines, is relatively expensive. Several machines, developed for aquatic weed control, can be used to remove H. verticillata plants from irrigation canals and drains; these include mowing buckets attached to a tractor or hydraulic excavator.

Different harvesters can be used to remove H. verticillata from lakes; these harvesters collect the plant material and dump it on the shore. One problem in the use of these harvesters is that cuttings of hydrilla, which are not removed from the water, help to spread the weed.

In the USA, lake drawdowns are occasionally used to expose the plant and dry it out (USDA, 2011).

Chemical Control

The application of herbicides in or near waterbodies may have serious consequences for the environment and can endanger the health of local people if the water is used for drinking, swimming or washing. As a consequence, there are strict regulations for the use of chemicals to control aquatic weeds in many countries. The risk is greatest when the herbicide is introduced directly into the water, which is necessary for the control of submerged weeds.

There are many chemicals which are effective against H. verticillata but only a few of these compounds are reasonably safe for the environment and human health. One additional problem is that the use of herbicides can lead to the build up of masses of decaying plant material and, as a consequence, a sudden decrease in the oxygen content of the water.

Until recently, the main herbicide options for H. verticillata were endothal, diquat and copper. The costs of applying these contact herbicides twice a year in Florida amounted to approximately $400 per ha (Haller, 1995).

A systemic herbicide, fluridone, has been developed which can provide effective control of hydrilla; this control usually lasts for about one year. The costs amount to approximately $200 per ha per year (Haller, 1995). The herbicide, bensulfuron-methyl, has recently been tested against H. verticillata (Van and Vandiver, 1994). After one month, effective concentrations caused severe damage but regrowth occurred rapidly where herbicide exposure was limited to periods of less than 14 days.

Biological Control

Biological control is theoretically the best method of controlling aquatic weed problems because the effect is lasting and relatively inexpensive. The most promising biological control agent for H. verticillata is the phytophagous fish, grass carp or white amur (Ctenopharyngodon idella). It feeds on many aquatic plants, in particular, submerged species. It is presumed that the grass carp will not breed outside its native habitat in China and Siberia as it requires special conditions for spawning. However, artificial reproduction is possible in fishery stations. In the USA, 180-320 kg/ha of grass carp is required for effective control of H. verticillata, whereas 80-70 kg/ha is sufficient in India (Pieterse, 1981).

A sterile, triploid grass carp has been bred because there is some controversy in the USA over the possibility of grass carp reproducing naturally in sufficient quantities to interfere with fisheries and waterfowl populations. This hybrid grass carp is the result of a cross between a female grass carp and a male bighead carp (Hypophthalmichthys nobilis). Control of H. verticillata using grass carp and hybrid grass carp is highly economical. In the USA, this control amounts to approximately $12 per ha per year (Haller, 1995). However, the fish needs to be retained within a certain area using fencing to prevent movement from the target site. This also allows the fish to be removed if the population needs to be managed.

Four insects of the genera Bagous and Hydrellia have recently been introduced into the USA by entomologists. These insects have eliminated large infestations of H. verticillata. Bagous affinis from India was released in Florida, USA, in 1987 (O'Brien and Pajni, 1989). This species was recently shown to have potential as a biological control agent of H. verticillata in California as it survived the winter season (Godfrey et al., 1994). Research on these insects and other potential candidates for biological control continues.

For a full account of the results of recent biological control introductions, see Center (1992).

Integrated Pest Management

An integrated approach to the control of H. verticillata has been followed in Fish Lake in Florida, USA, an area which consists of 22 lakes in the headwaters of the Kissimmee River. This approach involves applying the herbicide, fluridone, and subsequently releasing hybrid grass carp. The weed has been sufficiently controlled for four years and the integration of these methods may reduce herbicide applications to once in 5 years (Feller and Bodle, 1995).
 

 

References

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